How difficult is it to track people who overstay their visas? It might seem easy to match visas coming into the United States with visas going out, but the reality has proved much different.

In April 2011, the Department of Homeland Security had a backlog of 1.6 million “overstays”: people who arrived on temporary visas and were unaccounted for at the time of their expected departure. High-profile cases have brought the issue to attention. Two of the conspirators in the 1993 World Trade Center attacks had overstayed visas. More recently, students from Kazakhstan linked to the Boston bombers were living in the United States on expired or invalid student visas.

In summer 2011, Homeland Security had completed a review of the 1.6 million overstays. It found that 863,000 had left or were otherwise accounted for. Of the remaining, it identified 1,901 subjects who could pose national security or public safety risks. The Government Accountability Office said key challenges still exist, particularly in starting a biometric program for departures at airports and seaports that would analyze such characteristics as fingerprints and eyes.

Among its challenges: who would be responsible for obtaining the information and how the timeliness of departures would be affected. The Senate Judiciary Committee recently approved an amendment to require biometric exits at the 10 busiest U.S. airports within two years.

Writing for the Hill congressional newspaper last week, Edward Alden, a senior fellow at the Council on Foreign Relations, said critics have exaggerated the overstay problem, pointing out that the number of overstays has plummeted. And because most U.S. airports are not configured to do the biometric screening, he said, a better alternative is an existing program in which Homeland Security gathers biographic information on departing passengers and matches it with entry records.

“U.S. tracking of visa overstays is not perfect, but neither is it the massive hole in immigration enforcement that too many in Congress believe exists,” he said.